The present invention relates generally to the construction and use of intravascular catheters and, more specifically, to catheters which may be used both as a guidewire for positioning over-the-wire catheters and as a Doppler interferometric imaging catheter for intravascular diagnosis of atherosclerosis.
Atherosclerosis is a common human disease characterized by the deposition of fatty substances (atherosis) in, and a fibrosis of the inner layer of the arteries (sclerosis). Accumulated in blood vessels, these depositions restrict the blood flow and place a person's health at serious risk.
Multiple interventional therapeutic techniques have been developed recently for reduction or elimination of obstructing plaque and restoration of the patency of the vessel. Among these techniques are: balloon dilation, tip abrasion, atherectomy, laser ablation, placing stents and thrombolytic therapy.
Atherosclerotic lesions can be of different geometries, degree of progression, and/or complicated with calcification and thrombi. None of the mentioned techniques is universal, each of them has it's own situational advantages and contra-indications, so different types of atherosclerotic lesions require different therapeutic interventions. To make an educated choice of an appropriate therapeutic modality and specify the dimensions of the therapeutic tooling, such as diameter, length, etc., it is necessary to have complete diagnostic information about the lesion, which includes its geometry and composition. But present imaging technology severely limits the ability of cardiologists to get this information prior to intervention and to guide intervention in real time.
There is a strong unsatisfied demand for an advanced imaging system capable of providing adequate information in diagnostic support and guidance of intravascular intervention, especially in the most widely used interventional technique: Percutaneous Transluminal Coronary Angioplasty (PTCA).
The most commonly employed modality for vascular imaging is angiography. It provides two dimensional blood flow mapping. A radiopaque dye is injected into the vessels being visualized and X-ray fluoroscopy indicates the path of the dye flow as a dark projection path on a white background. It enables an interventionalist to find the location of the stenotic area and assists in inserting guidewires and catheters into the diseased vessel for performing PTCA. Angiography shows the boundaries between the blood and inner surface of the vessel and has very limited ability to provide information about composition of intravascular obstructions and their geometry. Angiography is also used for examining the final patency of the vessel, and overall assessment of the outcome of the PTCA procedures.
Angioscopy supplies visual information on the luminal surface. To function, angioscopy requires flushing the blood from the vessel and replacing it with optically transparent saline. This special procedure requires temporarily occluding the vessel, stopping the blood supply to the heart and can cause prolonged ischemia. Because of this drawback, angioscopy is used very rarely, mainly for research purposes.
Laser induced fluorescence (LIF) employed for differentiation of diseased and healthy tissue so far has had very limited success. It does not provide geometric information about the diseased vessel and it's ability to guide a therapeutic procedure is questionable. Similarly, this is true of many of the newer proposed spectroscopies, such as Raman, or Plasma emission.
The intravascular ultrasound imaging technique (IVUS) has been developed in the past several years. In this approach an ultrasound emitter and a receiver are placed inside an artery and are supplied with a means to rotate the beam around the vessel.
Reflected ultrasound energy from the surface of the lumen and it's subintimal structures is collected by the receiver and processed by a computer. A cross-sectional reflectance image is thus generated and displayed on a video monitor. The architecture of the vessel, atherosclerotic plaque and calcification can be observed. But as recent studies have demonstrated, IVUS lacks several crucial features: 1) It does not provide some very important information, for example, discerning thrombus from soft atheroma (fatty lesion); 2) IVUS catheters are big and cannot be used simultaneously with a therapeutic catheter inside the artery--for diagnosis and evaluation of the results of the intervention it takes additional procedures to insert them into the artery and pull them back before and after the therapeutic procedure; and 3) They are relatively expensive. To meet hospital requirements for standard equipment of that kind, they should be significantly cheaper.
A known imaging catheter is described in Pomeranz U.S. Pat. No. 5,095,911. The device employs an ultrasound transducer and rotating mirror to scan the interior of a vessel. A drive cable which is driven from outside the patient causes the transducer and the reflective surface to rotate as a unit. The catheter body is formed of a flexible wire coil coated with a polymeric material. The catheter body is flexible enough to manipulate within the patient's vascular system, but has torsional stiffness sufficient to allow for rotation to position and guide the catheter through the vascular system. The outer diameter of the catheter body, according to the author, and the end containing the ultrasound transducer is between about 0.3 and 1 millimeter. However, there are currently no known ultrasound transducers having outer diameters which are smaller than 0.8 millimeters.
Projecting from an end of the housing containing the ultrasound transducer, opposite the end connected to the catheter body is a fixed guidewire with a deviated tip.
The guidewire portion is used to position the ultrasound transducer probe for imaging. The probe as well as the catheter body are much larger in diameter than the diameter of the guidewire. The diameter is too large to use in connection with most over-the-wire therapeutic catheters, in particular, catheters used in PTCA procedures, which require an 0.35 millimeter or smaller outer diameter guidewire. In addition, the therapeutic catheter should be moved away from the lesion and the probe must be moved back and forth across the diseased tissue to form an image between interventional actions.
Angelsen et al. U.S. Pat. No. 4,887,605 describes a laser catheter which includes ultrasound imaging capability. At the distal end of the catheter is a housing which contains an ultrasound transducer for emitting an ultrasonic beam. A cylindrical housing is provided having a central throughbore. An optical fiber is positioned in the throughbore and is used to deliver a beam from a laser to the tissue to be treated. The catheter described in this patent is intended to be used as a diagnostic and therapeutic catheter, and is not intended to function as an imaging guidewire. Furthermore, by combining the diagnostic means with the therapeutic means, the user has sacrificed the ability to choose the most appropriate therapeutic treatment for a given situation. The combination of these two means would imply that the choice of therapy is made without prior knowledge of the geometry and morphology of the diseased vessel.
Another known ultrasound imaging and laser catheter is described in Morantte, Jr. U.S. Pat. No. 4,587,972. This catheter includes a bundle of optical fibers extending through a cylindrical body for delivering a laser beam. The device also includes an ultrasound transducer at its distal end for imaging vascular obstructions. The distal end contains a number of electrodes which generate ultrasonic waves when energized that travel axially of the cylindrical body, providing a scanning angle of 30-45 degrees. This device has the same drawbacks as the device described in U.S. Pat. No. 4,887,605.
Using current technology, it is not possible to reduce the size of an ultrasound probe small enough to pass through a central cavity of a therapeutic catheter, such as a balloon catheter. In order to incorporate such a probe into a distal end of a guidewire, a housing of the ultrasound probe would need to have a maximum outer diameter of 0.35 millimeters. Such guidewires and some which are up to about 0.46 millimeters at the most, are currently in wide use in procedures such as PTCA.
The use of ultrasound imaging and angiography for geometric measurement have some distinct problems. The radiopaque dye used to identify the vessel boundaries penetrates diffused atherosclerotic lesions, causing the inner diameter of the diseased region of the vessel to appear larger than it actually is. Similarly, the sound waves emanating from an ultrasound transducer are not all reflected from the obstruction, creating an image with a blurred outline of the inner diameter of the vessel. The uncertainty in the measurement of a diameter of the blood vessel using either technique can be as high as 0.5 millimeters. Studies have shown that measurements taken on the same lesion from angiography and ultrasound do not correlate. The error is so large that the appropriate size therapeutic balloon catheter cannot be accurately determined.
Other imaging devices are known which are used as over-the-wire catheters. One such device is shown in Narciso, Jr. U.S. Pat. No. 5,217,456. An intravascular optical radial imaging system is described which utilizes the spectral qualities of tissue to identify the composition of the vessel. The main disadvantage of this device is that it requires removal prior to treating the vessel, and therefore the device is incapable of imaging during therapeutic treatment. The device also does not provide any geometric information about the vessel.
The use of low coherence interferometry for imaging biological tissue is generally known. PCT international publication WO 92/19930 describes a device which includes a low coherence light source that supplies light to two optical paths. The first path is for sampling and sends light to a probe. The second path is phase modulated and functions as a reference arm. The second optical path includes a mirror that traverses back and forth along a longitudinal path. The return reflective energy is combined and the output is converted into an electrical signal. The electrical signal is inputted into a computer which uses the information to create a visual image of the object being scanned.
Although this configuration provides optical imaging using interferometry, the amount of time needed for taking an image is too long for intravascular use. In order for intravascular imaging to be of maximum use during a procedure, the scanning process must take place within a fraction of a beat of the patient's heart. In addition, the device disclosed in international publication WO 92/19930 is incapable of being scaled down small enough to be utilized in an imaging guidewire that is compatible with PTCA instrumentation. The imaging device described in this reference is about 2 millimeters in diameter as compared to a guidewire outer diameter of 0.35 millimeters.
In P.C.T. international publication WO 9412095, a new imaging modality based on Low Coherence Interferometry is described. This approach to diagnostic imaging came into biology from the telecommunications industry, where it was employed for quality control of optical elements and fibers. In this approach light, instead of ultrasound, is used for diagnosis. A fiber, incorporated into the intravascular catheter is used for delivering light from an external source such as a superluminescent diode, to the tissue to be diagnosed, and for collecting the retroreflected light. The distal end of the catheter is provided with a mirror, brought into rotation from outside the body by an electrical motor.
A Michelson interferometer is used for analysis of the retroreflected light and for recovering the reflectance profile along the light beam. A movable mirror in the reference arm of the interferometer is moved in a sequence of positions, and the intensity of the interfering light is measured for each position, thus providing a plot of the reflected intensities as a function of depth into the tissue. A dithering of the reference mirror is combined with a phase locked detector to overcome 1/f noise. Rotation of the mirror at the distal end of the catheter allows the measurement of reflectance profiles (plots) for different directions around the vessel and to build a computer generated cross-sectional image of the vessel.
A serious drawback of this system employed for recovering the opto-electronic reflectance profile is that only while the reference mirror is at rest can the signal be recovered at all measuring points. The intensity at the output photodetector as a function of reference mirror position oscillates with a period of .lambda./2, as usual in those kinds of interferometric systems, and only the reconstructed envelope curve of the oscillation magnitudes follows the intensity profile of the reflected light. Thus to recover the retroreflected profile along 1 millimeter, it takes at least 2000-3000 points, and at each of these points the mirror should be stopped completely. Even with the fastest translation stage available, it takes at least several minutes to measure just one profile. To build the whole 360.degree. image, it takes another factor of 100, thus making the time of measurement of one cross-section above one hour. In other words, the system suggested cannot work in real time, it is too slow, because it is inherently incapable of working "on-the-fly", when measurement for any given position of the reference mirror can be done while the mirror is moving.
Another drawback is that the described device is an over-the-wire catheter and cannot be used simultaneously with a therapeutic device in the artery during intervention. It should be inserted before intervention, and then exchanged with a therapeutic catheter, thus significantly lengthening catheterization time.
All of the known devices have disadvantages which prevent them from being used in intravascular diagnosis. It would be desirable to have a high resolution guidewire based intravascular imaging device which can be used both for diagnostic imaging and for placement of over the wire catheters, providing diagnostic support and real time guidance for percutaneous transluminal coronary angioplasty and similar interventions.